Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique that uses orthogonal subcarriers to transmit information within an available spectrum. Because the subcarriers may be orthogonal to one another, they may be spaced much more closely together within the available spectrum than, for example, the individual channels in a conventional frequency division multiplexing (FDM) system. To help achieve orthogonality, a subcarrier may have a null at the center frequency of the other subcarriers. Orthogonality of the subcarriers may help prevent inter-subcarrier interference within the system. Before transmission, the subcarriers may be modulated with a low-rate data stream. The transmitted symbol rate of OFDM symbols may be low, and thus the transmitted OFDM signal may be highly tolerant to multipath delay spread within the channel. For this reason, many modem digital communication systems are turning to OFDM as a modulation scheme for signals that need to survive in environments having multipath reflections and/or strong interference. Many wireless communication standards have already adopted OFDM including, for example, the IEEE 802.11a standard, the Digital Video Broadcasting Terrestrial (DVB-T) broadcasting standard, and the High performance radio Local Area Network (HiperLAN) standard. In addition, several industry consortia, including the Broadband Wireless Internet Forum and the OFDM Forum, are proposing OFDM for fixed wireless access systems.
One problem with OFDM systems is that they may be more sensitive to phase noise and frequency offset relative to single carrier systems. Unlike single carrier systems, phase noise and frequency offset in OFDM systems introduce interference, including intercarrier interference and intersymbol interference. To demodulate the subcarriers, an OFDM receiver may perform a synchronization to determine the location of symbol boundaries and timing instants. Inaccurate synchronization may result in a reduced or loss of orthogonality between the subcarriers resulting intercarrier and/or intersymbol interference and reduction in system performance. Achieving accurate synchronization in an OFDM system may be more critical than in single carrier systems. Conventional OFDM systems have used a cyclic prefix and/or special OFDM training symbols to help achieve symbol timing and frequency synchronization, however channel effects, such as linear distortion, in a multipath channel make this a difficult task.
Another difficulty with OFDM systems is properly detecting an OFDM packet. Channel effects, for example, increase the probability of false packet detection, which may be referred to as a false alarm probability. A high false alarm probability may result in reduced system performance.